This article reports the effect of conditioning of dentinal surfaces on the sealing capability of a light-curing glass-ionomer restorative material, Fuji II LC. The in vitro evaluation measured gap formation at the tooth-restoration interface. The control specimens were restored with Fuji II LC after dentinal surface treatment with a polyacrylic acid solution, while the experimental specimens were restored without pretreatment. The sealing ability of the material to dentin was assessed by measuring the gap between the restorative material and tooth structure. The control group registered a mean marginal gap of 7.75 um, and the experimental group registered a mean gap of 10.75 um, but statistical analysis indicated that there was no significant difference between the two groups. Dentinal surface treatment prior to restoration with the light-curing glass-ionomer cement used in this study resulted in a smaller gap at the dentin-restoration interface, but this difference was not significant.